Kinetic studies of tumor cells from children with leukemia, lymphoma, and neuroblastoma are being conducted in order to define the proliferative characteristics of human malignant cells and to use this information for the design of improved treatment. Studies of 1) the mitotic index, 2) in vitro (3H)dThd autoradiographic labeling index and 3) DNA histograms obtained by flow microfluorometry of propidium-iodide stained samples, before and sequentially following drug treatment are correlated with clinical characteristics and outcome. Kinetic study of tumor cell populations, initially and in the steady-state, defines patterns of cell proliferation present and contributes to an understanding of factors regulating growth.